1. Technical Field
The invention relates to linear arrays of loudspeakers.
2. Description of the Problem
Large space, public sound systems rely on a combination of loudspeaker types to achieve efficiency, wavefront coherence, a broad and level audio frequency bandwidth and good coverage of an audience located in the space. A foundational element in many such public sound systems is a linear array of multi-transducer loudspeakers. A linear array (sometimes termed a “line array”) is, in its classical form, a vertical row of closely spaced, cone type, direct-radiator acoustical drivers set in a baffle. Adjacent acoustical drivers are spaced to be mutually coupled in operation. This results in compression of the resultant sound field vertically and increases directivity of the sound field. The array can then be aimed to direct the sound field reducing the amount of energy lost to areas of less interest.
In a classic linear array the mutual coupling of the acoustic drivers results from the acoustic drivers being physically identical to one another, being used to produce the same sounds and being closely spaced. What constitutes “closely spaced” is a function of the highest audio frequency that the array is intended to produce, but roughly means that the center of each speaker cone should be spaced from the centers of adjacent cones by no more than a quarter wavelength of the highest frequency sound the array is intended to reproduce. Audible sound ranges in wavelength from about 17 meters at 20 hertz to 1.7 cm at 20 Kilohertz. The smallest direct radiator cone type speakers used are usually on the order 10 cm in diameter which allows spacing for good sound reproduction up to a frequency of about 3 Kilohertz. This provides for good speech intelligibility. The use of ribbon or planar type devices (which function essentially as linear arrays with zero spacing between elements) can be used for higher frequencies allowing higher quality reproduction of music.
While the vertical compression of the directivity pattern of the sound field generated by a linear array has long been recognized, less benefit has been obtained horizontally. Typically, the directivity pattern for a linear array has exhibited a shotgun pattern, with substantial amounts of energy being directed into two or more side-lobes. It would be desirable to generate a sound field with a directivity pattern exhibiting a cardioid shape thus directing more sound into an area corresponding with a more typical dispersal of an audience.